Coupling of FeS2-xSex nanoparticles and foam-like carbon superstructure to achieve high-density electron flow for enhanced sodium storage performance

The advantage of FeS 2 in sodium storage is constrained by its poor inherent conductivity and unstable electrochemical activity. In this work, the hierarchical structure, FeS 2-x Se x nanoparticles with high activity encapsulated into the foam-like carbon network, are fabricated by in-situ chelation/self-crosslinking and synchronous sulfurization/selenization strategies. Inside, the incorporation of Se into FeS 2 markedly changes the local electron distribution around the Fe atoms and improves the intrinsic conductivity of FeS 2 . Outside, S, Se codoped foam-like carbon networks are evenly wrapped on the surface of FeS 2-x Se x nanoparticles, which accelerate the diffusion of internal charges into the electrolyte. Excitingly, the Se-replaced FeS 2-x Se x nanoparticles and carbon shell established an unobstructed pathway through robust Fe-S-C and Fe-Se-C bond connections, realizing the transmission of more charge transfer, and finally forming a high-density electron flow. This powerful charge transfer mode from the inside out realizes an overall conductivity improvement of the materials and markedly enhances the rate performance. Moreover, the obtained FeS 2-x Se x nanoparticles ensure high electrochemical activity, and the external carbon layer effectively manages the volume expansion during the sodium storage process. As expected, the FeS 2-x Se x @SSe/C(5:1) exhibits a high capacity (603.73 mAh g −1  at 1 A g −1 over 600 cycles), outstanding rate capability, and cycle performance (225 mAh g −1  at 3 A g −1 over 5231 cycles) for sodium-ion batteries.